Experimental Study on the Limits of Flow Regimes for Different Configurations of Stepped Spillway

Transcription

1 Civil and Environmental Researh Experimental Study on the Limits of Flow Regimes for Different Configurations of Stepped Spillway Najm Obaid Salim Alghazali 1*, Salam M. Jasim 2 1. Asst. Prof. Dotor, Civil Engineering Department, Babylon University, Iraq. 2. M.S. Student, Civil Engineering Department, Babylon University, Iraq. * of the orresponding author: dr.nalghazali@gmail.om Abstrat Flow over a stepped spillway an be divided mainly into three flow regimes that are nappe, transition and skimming depending upon the disharge and the dimensions of the stepped spillway. The determination of the flow regime is a very important aspet in the design of stepped spillway due to the different properties for eah flow regime. Experimental work on flow regime limits for stepped spillways has been onduted using twelve stepped spillway models. The models were manufatured with three downstream slope angles: 25, 35 and 45, and four numbers of steps: 5, 10, 15 and 20. Five onfigurations of steps were tested, whih are onventional Flat, pooled, porous end sills, pooled with gabions and porous end sills with gabions. The results revealed that the end sills highly affet flow regime type; this effet is primarily for the lower limits of skimming flow. Using end sills inreases the range of transition flow regime (by inreasing the lower limit of skimming flow) as well as inreases the instabilities that our in this flow regime. Gabions redue the effets of end sills on the lower limit of skimming flow regime to near the limit of flat steps. New empirial equations were suggested based upon the experimental results. Keywords: Flow Regimes, Gabions Steps, Nappe Flow Regime, Pooled Steps, Skimming Flow Regime, Stepped Spillway, Transition Flow Regime. 1. Introdution The stepped spillway is a spillway whose fae is provided with a series of steps from near the rest to the toe, they have gained popularity with modern onstrution tehniques inluding roller ompated onrete (RCC) and gabions (Chanson 2001, Felder and Chanson 2013, Guenther et al. 2013) In the last two deades, there has been an inreasing interest in the stepped spillways in various laboratories around the world (Khatsuria 2005). The stepped spillway design is not limited to flat uniform steps, some prototype stepped hutes were designed with pooled steps (e.g. Sorpe dam, Germany), alternate sills (e.g. Neil Turner stepped weir, Australia) and weir strutures designed with gabion steps, et. (Chanson and Gonzalez 2004, Felder and Chanson 2013), the alternative stepped designs are poorly understood (Felder and Chanson 2013). In reent years, the flows on pooled stepped spillways were researhed in a few studies (Felder et al. 2012). Also, the hydraulis of gabions stepped spillway has reeived less attention due to the omplexity to evaluate the flow patterns and flow resistane (Chinnaarasri et al. 2008). The advantages of gabions are: low ost, ease of installation, flexibility, and ease of maintenane (USACE 1986). With proper onstrution pratie, spillways having a stepped downstream fae built of gabions an withstand floods of up to 3 m 2 /s without damage (Peyras et al. 1992). Stone size and shape have little influene on the energy loss and flow veloity as ompared to the inreasing effet of the weir slope (Chinnaarasri et al. 2008). Many relationships have been developed to predit the limits of flow regimes for onventional flat stepped spillway. The flow should be distintly either in the nappe flow or the skimming flow regime for the design disharge and the safety hek flood (Boes and Hager 2003). Due to the high instabilities and the impat load aused by the transition flow regime, researhers suggested a safety fator of 10-20% or even more to be added to the lower limit of skimming flow regime or subtrated from the upper limit of nappe flow regime determined by empirial equations (e.g. Boes 2012). Modern stepped spillways are designed for the skimming flow regime (Chanson 2001). The onservative design with the mentioned safety fator is suggested to avoid transition flow regime while it annot be avoided if the hute is designed for skimming flow for un-gated spillways (Boes and Hager 2003). This study aims to ontribute in the developing of a design guide for the limits of flow regimes on stepped spillways for five onfigurations of steps, whih are: onventional Flat, pooled, porous end sills, pooled with gabions and porous end sills with gabions. Another objetive is the introduing of onfigurations that may redue the transition 30

2 Civil and Environmental Researh flow instabilities and the hydrauli loads on the onrete fae of the RCC stepped spillways. 2. Experimental Work Twelve stepped spillway models were made from wood and oated with varnish to avoid wood swelling of water and to inrease its smoothness. The models have vertial upstream fae with three downstream slope angles (25, 35 and 45 ). For eah slope, four models were designed as ogee stepped spillway with 5, 10, 15 and 20 steps. All models have 0.3 m width and 0.3 m height (from the base to the upper point in the rest). The tests were arried out in a reirulating flume loated at the fluid laboratory of Engineering College, Babylon University, Iraq (Photo 1). The flume is 10 m length and 0.3 m width. It has transparent side walls with height of 45 m. The flume has a pump with a disharge apaity of 30 l/s, a flow meter is installed on its pipeline for measuring the disharge of the passing flow. Two movable arriages with point gauges were mounted on brass rail at the top of flume sides, whih have an auray of 0.1 mm. Photo 1: The used flume (Civil Engineering Department, Engineering College, Babylon University, Iraq). The experiments were onduted for fifteen disharges runs ranging from 0.9 to 9.3 l/s (Table 1). This range was satisfying the need of this study. The used end sills are of height equals to half step height i.e.: h; is the end sill height and h is the step height. Four thiknesses of end sills are used, that are: 0.5, 1, 3 and 5 mm for the models having number of steps of 5, 10, 15 and 20 steps respetively. The gabion dimensions are ( ( - ) ), where h e : is the end sill height, : is the step length, and t: is the end sill thikness. During testing runs, the gabions were plaed into the steps and removed alternately. Wire mesh of rhombus shape with side length of 0.68 m and diagonals of 0.65 and 1.2 m has been used. The wire mesh boxes were filled with gravel of size m and porosity of 41%. These types of wire mesh and gravel were seleted taking into aount that the filled material should be larger than 1.5 times the wire mesh opening and the porosity values between 38 and 42 are preferable as suggested by previous studies (suh as Stephenson (1979) and Kells (1993) ited in Salmasi et al. 2012). 31

3 Civil and Environmental Researh Table 1: Disharges used in the 15 runs. Run No. Q (l/s) q (l/s.m) Run No. Q (l/s) q (l/s.m) Observations Many observations are taken by using digital video-a ra Nikon D 1 (Fra advan ra : fps, S u r Speed: 1/4,000 to 30 s) in addition to the visual observations, these observations an be summarized as follows: 3.1. Pooled stepped models (non-porous end sills) have larger ranges of transition flow regimes rather than all the tested onfigurations. It seems that the end sills affet primarily the lower limit of the skimming flow regime. Photo 2: Flow r gi s on fla and pool d s pp d od ls (θ 45, 5 steps, run no. 9) Gabions redue the effets of end sills on the lower limit of skimming flow regime to near the limit of flat steps Photo 3: Flow regime and instabilities on pooled steps wi and wi ou gabions (θ 2, s ps, run no 9) 3.3. For porous end sills, there is an additional type of flow, whih is the flow through pores (Photo 4). This type of flow was inluded with the typial flow regimes. It an be identified visually for low disharges in nappe flow regime with a same behaviour. 32

4 Civil and Environmental Researh 4. Data Analysis 4.1 Upper Limit of Nappe Flow Regime Photo 4: Flow through pores. The equations seleted for omparing with the present study data are presented in Table 2 (NA refers to the nappe flow regime and SK is the abbreviation of skimming flow regime). Table 2: The studied relationships of flow regimes limits. Researher Equation Case Yasuda and Ohtsu (1999) Chanson (2001a) Chinnarasri (2002): 1 (1 an θ) 2 NA-Flat steps 9 ( ) NA-All ases 9 ( ) NA-Flat steps Chanson and Toombes (2004) Chinnarasri and Wongwises (2004) Yasuda and Ohtsu (1999) Chanson (2001a) Boes and Hager (2003) Chanson and Toombes (2004) Chinnarasri and Wongwises (2004) Chinnarasri et al. (2008) 91 1 NA-Flat steps 92 ( ) NA-All ases 1 ( ) SK-Flat steps ( ) SK-All ases 91 1 SK-Flat steps 9 21 SK-Flat steps ( ) ( ) ( ) 1 ( ) 2 1 SK-All ases SK-Gabion steps 33

5 Civil and Environmental Researh Suggested Relationships The following relationships are suggested for the upper limit of nappe flow regime on stepped spillways with downs r a slop angl 2 θ T r la ions ips ob ain d using Mirosof Ex l and S a is ia (v rsion 12) software programs. All symbols are defined in appendix 1. Table 3: Suggested empirial equations for the upper limit of nappe flow regime. Suggested Equations R 2 or R Case ( ) R 2 = Flat R= Pooled R= Porous end sills 19 9 R= Pooled with gabions 2 2 R 2 = Porous end sills with gabions A Comparison with Previous Relationships The satter diagrams of the urrent study data and the data obtained from the relationships in Table 2 are shown in the following figures: Figure 1: Nappe flow regime upper limit-flat steps. 34

6 Civil and Environmental Researh Figure 2: Nappe flow regime upper limit-pooled steps. Figure 3: Nappe flow regime upper limit-pooled with gabions. Figure 4: Nappe flow regime upper limit-porous end sills. 4.2 Lower Limit of Skimming Flow Regime Suggested Relationships Figure 5: Nappe flow regime upper limit-porous end sills with gabions. The following relationships are suggested for the lower limit of skimming flow regime on stepped spillways with downs r a slop angl 2 θ, r la ions ips ob ain d using Mirosof Exel and Statistia (version 12) 35

7 Civil and Environmental Researh software programs. Table 4: Suggested Equations R 2 or R Case R= Flat R 2 = Weak orrelation between data Weak orrelation between the data Weak orrelation between the data Pooled Pooled with gabions Porous end sills Porous end sills with gabions Suggested relationships for lower limit of skimming flow A Comparison with Previous Relationships The satter diagrams of the urrent study data and the data obtained from the relationships in Table 2 are presented in the following figures: Figure 6: Skimming flow regime lower limit-flat steps. Figure 7: Skimming flow regime lower limit-pooled steps. 36

8 Civil and Environmental Researh Figure 8: Skimming flow regime lower limit-pooled with gabions. Figure 9: Skimming flow regime lower limit-porous end sills. 5. Conlusions Figure 10: Skimming flow regime lower limit-porous end sills with gabions The determination of the flow regime on stepped spillway is a very important aspet in the design of stepped spillway due to the different properties for eah flow regime. Experimental work was performed using twelve stepped spillway models to study flow regime limits for stepped spillways. The models were manufatured with three downstream slope angles: 25, 35 and 45, and four numbers of steps: 5, 10, 15 and 20. Five onfigurations of steps were tested, whih are onventional Flat, pooled, porous end sills, pooled with gabions and porous end sills with gabions. The results of experimental work were used to form new empirial equations for the limits of flow regimes for the onfigurations tested. It an be onluded that the end sills highly affet flow regime type; this effet is primarily for the lower limits of skimming flow. Using end sills inreases the range of transition flow regime (by inreasing the 37

9 Civil and Environmental Researh lower limit of skimming flow) as well as inreases the instabilities that our in this flow regime. Gabions redue the effets of end sills on the lower limit of skimming flow regime to near the limit of flat steps. 6. Referenes [01] Boes, R. and Hager, W. (2003). "Hydrauli Design of Stepped Spillways." J. Hydr. Eng., ASCE, 129 (9), [02] Chanson, H. (2001). "The Hydraulis of Stepped Chutes and Spillways." Balkema, Lisse, the Netherlands. [03] Chanson, H. and Gonzalez, C.A. (2004). "Stepped Spillways for Embankment Dams: Review, Progress, and Development in Overflow Hydraulis." Pro. Intl Conf. on Hydraulis of Dams and River Strutures, Tehran, Iran, Balkema Publ., The Netherlands, pp [04] Chanson, H. and Toombes, L. (2004). "Hydraulis of Stepped Chutes: The Transition Flow." J. Hydr. Res., IAHR, 42(1): [05] Chinnaarasri, C. and Wongwises, S. (2006). "Flow Patterns and Energy Dissipation over Various Stepped Chutes." J. Irrigation and Drainage Eng., ASCE, 132 (1): [06] Chinnaarasri, C., Donjadee, S. and Israngkura, U. (2008). "Hydrauli Charateristis of Gabion-Stepped Weirs." J. Hydr. Eng., ASCE, 134(8): [07] Felder, S. and Chanson, H. (2013). "Air Entrainment and Energy Dissipation on Porous Pooled Stepped Spillways." International Workshop on Hydrauli Design of Low-Head Strutures (IWLHS), Aahen, Germany, (87-97). [08] Felder, S., Guenther, P. and Chanson, H. (2012). "Air-Water Flow Properties and Energy Dissipation on Stepped Spillways: A Physial Study of Several Pooled Stepped Configurations." Researh Report No. CH87, Shool of Civil Engineering, the University of Queensland, Brisbane, Australia. [09] Guenther, P., Felder, S. and Chanson, H. (2013). "Flat and Pooled Stepped Spillways for Overflow Weirs and Embankments: Cavity Flow Proesses, Flow Aeration and Energy Dissipation." IWLHS. [10] Khatsuria, R.M., (2005). "Hydraulis of Spillways and Energy Dissipators." Marel Dekker, New York, U.S.A., pp [11] Peyras, L., Royet, P. and Degoutte, G. (1992). "Flow and Energy Dissipation over Stepped Gabion Weirs." J. Hydr. Eng., ASCE, 118(5): [12] Salmasi, F., Sattar, M. and Pal, M. (2012). "Appliation of data mining on Evaluation of Energy Dissipation over Low Gabion-Stepped Weir." Turkish Journal of Agriulture and Forestry No. 36: [13] Sarfaraz, M. and Attari, J. (2011). "Seletion of Empirial Formulae for Design of Stepped Spillways on RCC Dams." World Environmental and Water Resoures Congress, ASCE: [14] USACE (1986). "Use of Gabions in the Coastal Environment." CETN-III-31, 12/86. 38

10 Civil and Environmental Researh Appendix 1: Symbols Symbol Unit Definition Fr * [ ] Roughness Froude number g (sin θ)( os θ) g [m/s 2 ] Gravity aeleration h [m] Step height l [m] Step width h e [m] End sill height Q [m 3 /s] Disharge q [m 2 /s] Disharge per unit width R 2 [ ] Coeffiient of determination y [m] Critial flow depth above spillway rest θ Degree Downstream slope angle [m] Tan -1 ( ) (it is zero for a horizontal step). 39

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